Current and future communication systems will require MIMO antenna systems capable of operation over multiple frequency bands. Isolation between adjacent elements as well as de-correlated radiation patterns will need to be maintained across multiple frequency bands, with antenna efficiency needing to be optimized for the antenna system.
In many instances, the line of sight between a transmitter and a receiver involved in the communication becomes blocked or shadowed with obstacles such as walls and other objects. Each signal bounce may introduce phase shifts, time delays, attenuations and distortions, which ultimately interfere at the receiving antenna. Destructive interference in the wireless link is problematic and results in degradation of device performance.
A signal quality metric is often used to assess the quality of signals. Examples of such quality metrics include signal-to-noise ratio (SNR), signal to interference-plus-noise ratio (SINR), receive signal strength indicator (RSSI), bit error rate (BER) and other metrics, which are called channel quality indicators (CQI). Multiple Input Multiple Output (MIMO) systems or antenna diversity schemes can be used to improve the quality and reliability of a wireless communication link.
An antenna diversity scheme can mitigate interference from multipath environments by monitoring one or more CQIs. Antenna diversity can be implemented generally in several forms, including spatial diversity, pattern diversity and polarization diversity. Each of these diversity schemes requires one or more processing techniques, such as switching, selecting and combining.
Switching is one of the simple and efficient processing techniques and generally includes receiving a signal from a first antenna until the signal level fades below a threshold, at which point active components such as switches engage the second antenna for communication with the receiver.
Selecting is a processing technique that determines an optimal signal for utilization by the receiver during each predetermined time interval. Both selecting and switching techniques may utilize active components, such as switches, to select the optimal signal based on one or more CQIs. The selecting and switching techniques may be collectively called a switching technique wherein the selection of the signal for utilization is carried out by controlling the switches or other active components coupled to the antennas.
Combining is a processing technique wherein the multiple signals at the receiver are weighted and combined into a signal for communication. The weights are adjusted such that a main lobe of a receiving beam pattern is toward the desired direction and a null of the receiving beam pattern is toward the interference direction. Examples of combining techniques include a minimum mean squared error (MMSE) combining technique, a maximum ratio combining (MRC) technique and an equal gain combining (EGC) technique.
In the diversity scenarios, the different signals received at the different antennas of the receiver are coming from the single transmitting antenna and contain the same message. The selecting or switching schemes may be adapted for Single Input Single Output (SISO) systems while the combining scheme may be adapted for Multiple Input Single Output (MISO) systems.
Different from diversity schemes, a MIMO system can mitigate interference from the multipath environment by using several transmit (Tx) antennas at the same time to transmit different signals, which are not identical but are different variants of the same message, and several receive (Rx) antennas at the same time to receive the different signals. A MIMO system can generally offer significant increase in data throughput without additional bandwidth or increased transmit power by spreading the same total transmit power over the antennas so as to achieve an array gain. MIMO protocols constitute a part of wireless communication standards such as IEEE 802.11n (WiFi), 4G, Long Term Evolution (LTE), WiMAX and HSPA+.
However, in a MIMO system, each radio link between one of the Rx antennas and one of the TX antennas may still face destructive interferences due to phase shifts, time delays, attenuations, distortions and various other electromagnetic effects as explained earlier. It is possible to improve the overall performance of the MIMO system by improving the quality and reliability of each link by using a selecting or switching diversity scheme, for example.
In Rx diversity for a conventional wireless mobile device, one or more diversity antennas are added in the device to support the diversity scheme. A MIMO system already uses multiple (N) antennas; thus, if each of the Rx antennas in the MIMO system needs one diversity antenna, the system would end up containing 2×N Rx antennas. In such a configuration with multiple antennas, size constraints may become significant, and coupling among the antennas as well as between the antennas and nearby electronics of a communication device may significantly deteriorate transmission and reception qualities. Additionally, efficiency may deteriorate in many instances where multiple paths are energized and power consumption increases. Implementing two, three or more diversity antennas may provide system flexibility, but the volume and area requirements become increasingly severe as additional antennas, associated components and transmission lines are needed. As such, mixing diversity and MIMO techniques has gained limited popularity thus far.
With Multiple Input Multiple Output (MIMO) systems recently becoming more prevalent in the cellular communication field, the need for two or more antennas collocated in a mobile device are becoming more common. These groups of antennas in a MIMO system need to have high, and preferably, equal efficiencies along with high isolation and low correlation. For handheld mobile devices the problem is exacerbated by antenna detuning caused by the multiple use cases of a device: hand loading of the cell phone, cell phone placed to user's head, cell phone placed on metal surface, etc. For cell phone applications, the multipath environment is constantly changing, which impacts throughput performance of the communication link.
The 3GPP LTE standard specifies five Categories of operation, each Category providing a different data rate for both uplink and downlink communication. The data rate increases as operation progresses from Category 1 to Category 5. Category 1 allows for SISO (Single Input Single Output) operation, which is defined as one antenna on the base station side of the communication link, and one antenna on the mobile side of the link, for example. Categories 2, 3, and 4 specify a maximum of a 2×2 MIMO antenna system, with two antennas on the base station side and two antennas on the mobile side. Category 5 specifies a maximum of 4×4 MIMO antenna system, with four antennas on the base station side and four antennas on the mobile side. It is important to reiterate that though most of the categories of operation for LTE communication systems require a MIMO antenna system, i.e. multiple antennas, Category 1 operation is specified in the standard which allows for single antenna operation if minimum data rates can be achieved. For a majority of cases, such as low signal strength regions, high multipath environments and issues involving deep signal fading, a single antenna will not provide the data rate required for Category 1 operation. In this case, additional antennas and communication ports are activated to improve system throughput.
Category 5 operation allows for a maximum of a 4×4 MIMO antenna system, with this Category providing the highest data rate of the five Categories. The problems associated with implementing four antennas in a mobile device for use in a cellular network are numerous: making volume internal to a mobile device available for four antennas, providing a four port transceiver configured for MIMO operation, the power consumption of a four port solution compared to a two port solution, and the difficulty of integrating four antennas into a mobile device while maintaining high isolation and low correlation between the antennas.